The molecule beta-lipotropin, composed of 91 amino acids (beta-LPH 1-91) has gained considerable importance in recent years. Its double role as the precursor of beta-MSH (beta-LPH 41-58) and beta-endorphin (beta-LPH 61-91) makes this peptide unique in its kind. Results are presented on the role of this molecule and on the complete characterization of two morphine-like peptides from human and sheep pituitaries. The structure-activity relationship of opiate activity is analyzed by scanning for this biological activity of many tryptic and CNBr fragments of beta-lipotropin. The unequivocal localization of one of the important synthesis sites of beta-endorphin in the pituitary neurointermediate lobe is presented. A peptide with partial sequence Met1, Leu8,15 and Lys6,11, 27, 29, 33 has been biosynthesized in large quantities and its ubiquitous nature and conservation of sequence speaks for its importance and possible presence in many living cells. This polypeptide was subsequently identified as ubiquitin, a non-histone fragment of the nuclear protein A-24.